1. Field of the Invention
The present invention relates to a semiconductor device having a circuit formed by a thin film transistor (hereafter referred to as a TFT), and to a method of manufacturing the semiconductor device. For example, the present invention relates to an electro-optical device, typically a liquid crystal display device, and to electronic equipment provided with the electro-optical device as a part. Note that, throughout this specification, the term semiconductor device indicates general devices which function by utilizing semiconductor characteristics, and that the above electro-optical device and electronic equipment fall under the semiconductor device category.
2. Description of the Related Art
Techniques of crystallizing and increasing crystallinity of an amorphous semiconductor film formed on an insulating substrate such as glass by performing heat treatment, laser annealing, or both heat treatment and laser annealing have been widely researched in recent years. Silicon is often used in the semiconductor film.
Crystallized semiconductor films obtained in accordance with the above techniques are referred to as crystalline semiconductor films. The crystalline semiconductor films have extremely high mobility in comparison with amorphous semiconductor films. A monolithic type liquid crystal electro-optical device (a semiconductor device in which thin film transistors (TFTs) for a pixel driver and a driver circuit are manufactured on one substrate) which cannot be realized by a semiconductor device manufactured using a conventional amorphous semiconductor film, for example, can therefore be manufactured if a crystalline semiconductor film is utilized.
The crystalline semiconductor films are thus semiconductor films having extremely good characteristics compared to amorphous semiconductor films. This is why the above stated research is being carried out. For example, it is necessary to have a heat treatment temperature equal to or greater than 600° C., and a heat treatment time equal to or greater than 10 hours, preferably equal to or greater than 20 hours, when performing crystallization of an amorphous semiconductor film by using heat treatment. Substrates, which can withstand these crystallization conditions, include quartz substrates, for example. However, quartz substrates are high cost, and are lacking in processability. In particular, they are extremely difficult to be processed into a large surface area. Increasing the surface area of the substrate is indispensable for raising mass production efficiency, in particular. Work towards increasing the surface area of the substrate for increasing mass production efficiency has been remarkable in recent years, and a substrate size of 600×720 mm is becoming more and more of a standard for newly constructed mass production lines.
The processing of a quartz substrate into this type of large surface area substrate is difficult with present techniques, and even if it were possible, would not happen at present due to the costs or production. Glass is available, for example, as a material which can easily be manufactured into a large surface area substrate. A glass substrate referred to as Corning #7059 exists, for example, as this type of glass substrate. Corning #7059 is extremely low cost, has good processability, and is easily made into a large surface area substrate. However, Corning #7059 has a softening temperature of 593° C., and has a problem in heat treatment at 600° C. or higher.
Corning #1737 exists as one glass substrate with a relatively high softening temperature. The softening temperature is high at 667° C. If an amorphous semiconductor film is formed on a Corning #1737 substrate, and the substrate is then placed in a 600° C. atmosphere for 20 hours, there is almost no change in shape of the substrate which will influence manufacturing. However, a heat treatment time of 20 hours is too long in a mass production process, and from the point of view of costs, it is preferable to lower the heat treatment temperature of 600° C., even by a small amount.
A novel method of crystallization has been proposed in order to resolve these types of problems. This method is recorded in detail in Japanese Patent Application Laid-open No. Hei 7-183540. A simple explanation thereof is presented here. First, a very small amount of an element such as nickel, palladium, or lead is introduced into an amorphous semiconductor film. Methods such as plasma processing, evaporation, ion injection, sputtering, and liquid application can be utilized as the introduction method. Then, if the amorphous semiconductor film is placed, for example, in a 550° C. nitrogen atmosphere for 4 hours, a crystalline semiconductor film having good characteristics can be obtained. The optimal heat treatment temperature and heat treatment time for crystallization are dependent upon the amount of the element introduced and the state of the amorphous semiconductor film.
A method of crystallization of an amorphous semiconductor film in accordance with heat treatment is recorded above. On the other hand, the temperature of the substrate does not increase very much with crystallization by laser annealing, and high energy can be imparted to only the amorphous semiconductor film, and therefore substrates such as plastic substrates can also be used, in addition to glass substrates with low softening temperature.
Lasers such as an XeCl excimer laser, and a KrF excimer laser can be given as examples of the types of lasers which can be used in laser annealing. A method for performing laser annealing in which: a pulse laser beam from a high output excimer laser is processed into a square spot of several centimeters in size, or into a linear shape having a length equal to or greater than 10 cm, on an irradiation surface by an optical system; and in which the laser beam is then scanned (or the laser beam irradiation position is moved relatively with respect to the irradiation surface), has high productivity and is industrially superior. This method is therefore used preferably.
In particular, when a beam in which the shape of the laser beam is linear in the irradiation surface (hereafter referred to as a linear beam) is used, the entire irradiation surface can be irradiated by scanning the linear beam in only a direction perpendicular to the linear direction of the linear beam, differing from the use of a spot laser beam in which it is necessary to scan forward and backward, and left and right. Productivity is therefore high. Scanning in a direction perpendicular to the linear direction is performed because that is the most efficient scanning direction. This high productivity is the main factor in the present use pulse emission excimer lasers processed into a linear beam by a suitable optical system for laser annealing.
Further, there is also a method of performing crystallization of an amorphous semiconductor film by laser annealing after crystallization is performed in accordance with heat treatment. The characteristics of the semiconductor film may be made better when performing this method compared to performing only heat treatment or only laser annealing.